DC-DC converters are commonly used to supply DC power to electronic devices, such as personal computers, hand-held devices, and the like, and are available in a variety of configurations for deriving a desired DC output voltage from a given source of DC input voltage. For example, a buck mode or step-down DC-DC converter is often used to supply a regulated DC output voltage, whose value is less than the value of the DC source voltage. A typical step-down DC-DC converter includes one or more power switches, current flow paths through which are coupled between a DC input voltage terminal and a reference voltage terminal (e.g., ground), and the common or phase node between which is connected through an output inductor to an output voltage node, to which a storage capacitor and the powered load/device are connected. By controllably switching the power switches on and off, the upstream end of the output inductor is alternately connected between the DC input voltage and the reference voltage. This produces an alternately ramped-up and ramped-down output current through the output inductor to the output node, and causes a stepped-down DC output voltage to be delivered to the load. The DC-DC converter may be configured as a voltage mode converter or a current mode converter.
A voltage mode DC-DC converter, which is typically used in applications where load current demand is relatively large, includes a voltage control loop having an error amplifier, the output of which is used to control a PWM comparator, which generates a PWM voltage waveform. This PWM voltage waveform is applied to driver circuitry, which controls the turn on/off times of the power switches in accordance with times of transitions in respective PWM voltage waveforms with which it drives the power switches. To meet the demand for substantial load current, the PWM waveforms that control the on/off switching of the power switches are typically mutually complementary, so that a conductive path from one or the other of the input voltage source and ground will be continuously provided through one or the other power switch to the output inductor. This mode of operation is customarily referred to as continuous conduction mode (CCM). To regulate the DC output voltage, a voltage representative thereof is fed back to the error amplifier and compared with a DC reference voltage to produce an error voltage. This error voltage is amplified and filtered to produce an input signal to a PWM comparator, and is compared thereby with a sawtooth voltage waveform, to produce the PWM waveform, the pulse width of which is defined in accordance with the crossings of the error voltage threshold by the sawtooth voltage waveform.
A current mode DC-DC converter customarily includes an inner ‘current’ loop and an outer ‘voltage’ loop, which controls the inner current loop. The inner current loop contains a current amplifier, the output of which is coupled to comparator that is referenced to an error voltage provided by a voltage error amplifier in the outer voltage loop. As in the voltage mode DC-DC converter, the DC output voltage at the converter's output node is fed back to the error amplifier and compared with a DC reference voltage to produce an error voltage. This error voltage is used as a reference for the comparator of the inner current loop. The output of the comparator is coupled to reset a clocked flip-flop, complementary outputs of which are supplied to respective drivers for the power switches.
In addition to the above-described voltage mode and current DC-DC converters, there is an additional type of DC-DC converter, known as a constant on-time or pulse-frequency modulated (PFM) DC-DC converter. This type of converter is typically used in applications where load current demand is relatively small, as in the case of a “sleep” or “quiescent” mode of operation of a notebook computer, for example. A PFM converter includes a control loop having an error amplifier, the error voltage output of which is used as a voltage reference for a PFM comparator. The output of the comparator is coupled one or more drivers for the power switches. As in the voltage mode and current mode DC-DC converters, described above, the DC output voltage at the PFM converter's output node is fed back to the voltage error amplifier and compared with a DC reference voltage to produce an error voltage. In the control loop for the PFM converter, this error voltage is compared with a reference voltage by the PFM comparator, which outputs a triggering signal for a one-shot that sets a constant on-time for a relatively narrow pulse-width switching signal upon which switching times of the power switches are based. Because of its relatively narrow pulse-width, the switching signal provides the PWM mode converter with ability to turn on the power switches for very short time intervals—just sufficient to meet the very low current demands of the load, thereby saving power and prolonging battery life. This mode of operation is customarily referred to as discontinuous conduction mode (DCM).
In addition to PWM mode and PFM mode converters, described above, there is an additional hybrid architecture, known as a dual-mode, PWM/PFM DC-DC converter, that is designed to take advantage of the operational capabilities of each of PWM mode and PFM mode converters, by switching between PWM mode and PFM mode, depending upon load conditions. Namely, such a dual-mode converter employs PFM control when the output node is lightly loaded, and PWM control when the output node is heavily loaded. High efficiency is achieved by automatically selecting the more efficient mode of regulation, based on a continuous monitoring of the output current and the output voltage. However, a shortcoming of a conventional dual-mode DC-DC converter is the fact that a difference in initial conditions of the two regulation modes may cause the occurrence of a DC voltage anomaly in the output voltage at the time of switching between the two modes.